Power amplifiers are widely used for example in radio base stations and user equipments in wireless communication systems. Power amplifiers typically amplify input signals of high frequencies into an output signal ready for radio transmission. High efficiency is generally desirable for power amplifier design to reduce the amount of power consumed. Moreover, in many applications, such as in a satellite or a cellular phone or user equipment, the amount of power is limited due to powering by a battery. An increase in efficiency of the power amplifier would allow an increase of operational time for these applications before recharging or replacing the battery is necessary.
A conventional Power Amplifier (PA), such as Class B, AB, F, has a fixed Radio Frequency (RF) load resistance and a fixed Direct Current (DC) voltage supply. The RF output current of a Class B or AB PA has a form similar to a pulse train with half wave rectified sinusoid current pulses. The DC current, and hence the DC power, is largely proportional to the RF output current amplitude since the DC supply voltage is constant. The output power, however, is proportional to the RF output current squared. An efficiency of the conventional power amplifier, i.e. output power divided by DC power, is therefore also proportional to the RF output current amplitude. While efficiency of an amplifier may be high at the highest output power, the average efficiency is low when amplifying input signals that on average have a low power or low signal amplitude compared to the maximum input signal amplitude.
Some modified Class B RF power amplifiers, such as Doherty type amplifier disclosed in “A new high efficiency power amplifier for modulated waves,” Proc. IRE, vol. 24, no. 9, pp. 1163-1182, Sep. 1936, and Chireix type power amplifier disclosed in “High power outphasing modulation”, Proc. IRE, vol. 23, no. 2, pp. 1370-1392, Nov. 1935, are generally more efficient than the conventional amplifier described above for amplitude-modulated input signals with high Peak-to-Average Ratio (PAR). However Chireix and Doherty type amplifiers are inherently narrowband, since their operation depends on reactive circuits that are strongly frequency dependent.
Generally, RF power amplifier can be driven in a so called backed off operation. This means that the power amplifier is operated at a certain level, e.g. expressed as a number of decibels (dBs), under its maximum output power. Backed off operation may also refer to that an instantaneous output power is relatively low. When discussing operations of a power amplifier, the term “transition point” is generally used, which means that at a certain amplitude point, i.e. the transition point, some significant changes occur in the power amplifier, for example operating mode, number of active sub-amplifiers etc. In some amplifiers such as Doherty type, the transition points are also high-efficiency points in the efficiency plot.
Wideband Doherty type amplifiers are a subject of much interest, and many approaches have been attempted to increase the bandwidth and efficiency, which are unfortunately accompanied with some disadvantages or drawbacks.
For example, in a paper by D Gustafsson et al., entitled “Theory and design of a novel wideband and reconfigurable high average efficiency amplifier”, published in Proc. IMS 2012, a quarter wavelength transmission line with the same impedance is used as the load which results in wideband efficiency at the transition point. However, the wideband efficiency at the transition point is obtained by sacrificing efficiency at maximum power and utilization of transistor, which reduces the bandwidth of high average efficiency as well as increases the transistor cost.
In patent application WO2003/061115, filed by the present Applicant, a wideband amplifier with 100% relative bandwidth, i.e. frequencies at the high band edge and low band edge having a ratio of 3:1, and with high efficiency at backed off operation is disclosed. The wideband amplifier disclosed here has different operating modes in different frequency bands and has a wide instantaneous bandwidth around the centre frequency. However, it is difficult to operate across the borders of the frequency bands between the different operating modes. Further the output signal amplitude at the transition point varies considerably within the bandwidth.
In a paper by M Naseri Ali Abadi et al., entitled “An Extended Bandwidth Doherty Power Amplifier using a Novel Output Combiner”, published in Proc. IMS 2014, an LC-resonator is used at the output node. Using LC-resonator or using a resonant stub at the output node has the drawback of decreasing the output power bandwidth and efficiency bandwidth at full power.
In a paper by Piazzon et al., entitled “A method for Designing Broadband Doherty Power Amplifiers”, published on Progress in Electromagnetics Research, Vol. 145, pp319-331, 2014, or in a paper by R Giofrè et al., entitled “A Distributed Matching/Combining Network Suitable to Design Doherty Power Amplifiers Covering More Than an Octave Bandwidth”, published in Proc. IMS 2014, another technique involving the use of a multi-section branch line coupler is disclosed, which has limitation in the efficiency bandwidth both at the transition point and at full power, and also has limitation in the output power bandwidth at full power.